Drag/drop placeholder doesn't work if placeholder is smaller than draggable list elements

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
The placeholder should be placed correctly, regardless of if the placeholder is the same size as the list elements or a different size.

#### What is the current behavior?
The placeholder is in the wrong spot in the list while dragging an element, but only if the placeholder is smaller (less height for a vertical list, or less width for a horizontal list).

#### What are the steps to reproduce?
Stackblitz: https://angular-ztyby3.stackblitz.io
Drag the top element down. Observe that the placeholder moves to the top. Then, move it back upwards, and observe that the placeholder moves to the bottom. This is the opposite of how the placeholder should work.

#### What is the use-case or motivation for changing an existing behavior?
It does not work as expected.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@angular/*: 8.0.0
OS: Win10
TS: 3.4.5

#### Is there anything else we should know?

リンクされた StackBlitz の再現から始め、報告された Angular 8 のセットアップを使ってドラッグ＆ドロップの動作を再現します。次に、サイズの異なるリスト要素に対して placeholder を配置する Angular のドラッグ＆ドロップ実装を追跡します。要素を下方向にドラッグしてから上方向に戻したときに、要素がリスト要素より小さい場合も含め、placeholder が正しい位置に留まれば完了です。

索引モデルが issue の本文から書いたものです。